1. Field of the Invention
The present invention relates to a process for the preparation of a catalyst composite. Specifically, it relates to a process for the preparation of a catalyst composite material suitable for hydrocarbon conversion, and particularly, the fluid catalytic cracking of high vacuum gas oil feedstocks. The catalyst prepared by the process of this invention is highly active, and selectively reduces bottom and coke while increasing yields of gasoline and total cycle oil (TCO). Besides, the catalyst possesses improved attrition resistance.
2. Description of the Related Art
FCC is a major secondary refining process practiced worldwide in the petroleum refineries. FCC catalyst is the heart of this process providing considerable flexibility to refiners to meet the required product slates. To improve the efficiency of this process, it is desired to upgrade the bottom products (boiling above 370xc2x0 C.) to more valuable lighter products and less coke (due to regenerator hardware limitations). In this endeavor, not much emphasis is given in the prior art processes of FCC catalyst development.
Preparation of FCC catalyst is generally accomplished by admiring Y type zeolite, normal kaolin clay, and suitable binders. The produced aqueous slurry is generally subjected to agitation for uniform dispersion of individual components and then is spray dried to form dry catalyst microspheres.
A typical prior art catalyst preparation procedure involves the use of sodium silicate as silica source, aluminum sulfate, Y type zeolite, normal kaolin clay, and suitable acid to convert sodium silicate to silica sol binder. However, in this process, there are steps which involve repeated washing of sodium sulfate byproduct and subsequent exchange of sodium of the zeolite. This process is time consuming, hardware intensive, and requires a lot of demineralised water (DM). Also, there is a need for effluent treatment.
U.S. Pat. No. 4,142,995 mentions the use of silica-alumina gel in catalyst formulation for improving catalyst activity and attrition resistance. However, this art process has not addressed the issue of selectivity improvements like reduction of bottom and coke, and enhancement of TCO and gasoline yields. Person engaged in this art knows that preparation of silica-alumina gel is time consuming and requires several washings.
From an efficiency and economy point of view, it is generally preferred to use soda-free ingredients, i.e., binder, zeolite matrix, etc., to avoid series of washing steps. U.S. Pat. No. 4,443,553 teaches preparation of FCC catalyst using soda-free raw materials, wherein aluminium hydroxy chloride is used as viscosity reducing agent. This process suffers from the drawback, that is, chlorine evolved during the preparation procedure is corrosive and poses problems for the hardware and environment as well. Performance of the catalyst is not part of claims of the said patent. In addition to the above, presence of chlorine is likely to interfere with the efficiency of catalytic cracking.
While the prior art describes that different methods of formulating cracking catalysts often require high investments, catalysts are not able to meet the desired product requirements. Hence, the petroleum industry is continuously looking for a catalyst which will provide not only an acceptable range of physical properties and activity but also the most desirable selectivity with enhanced gasoline and TCO yields while reducing undesirable bottom and coke products. Development of bottom and coke selective catalyst becomes more relevant for low severity FCC unit operations prevalent in many countries like India.
Prior art processes demonstrate the performance of the steamed catalysts by means of ASTM Micro Activity Test (MAT), where the feed injection time is very long which leads to highly non-isothermal reaction conditions. The feed used is different from that used in commercial FCC units, and only one catalyst to oil ratio is employed. The product selectivity depends on these parameters and ASTM MAT is not suitable for predicting performance of the catalyst in a commercial FCC unit. To overcome this difficulty and to correctly predict catalyst performance in a commercial FCCU (FCC unit), in the present work, a commercial high vacuum gas oil feedstock is used to evaluate the catalyst at different catalyst to oil ratios at contact time much shorter than the conventional ASTM MAT conditions.
An object of the present invention is to provide a process for the preparation of FCC catalyst composite material useful for the conversion of petroleum hydrocarbons.
A further object of this invention is to provide a method for preparation of rare earth exchanged USY zeolite suitable for incorporation in FCC catalyst formulation.
It is another object of this invention to provide a FCC catalyst comprising modified alumina with silica.
It is yet another object of this invention to provide a process for the preparation of a FCC catalyst, which is simple and does not require any washing and exchange steps.
Yet another object of this invention is to provide a process for preparing a FCC catalyst having higher activity and properties to reduce coke and bottom yields.
Still another object of this invention is to provide a process for preparing a modified alumina component suitable for dual functionality of binding as well as enhancing catalytic activity.
It is still a further object of this invention to provide a process of catalyst preparation for increasing conversion, gasoline, yields and reduction of bottoms.
According to this invention, a process is provided for the preparation of catalyst, which consists of preparing a precursor slurry from highly dispersed components such as modified alumina-ammonium polysilicate, kaolin clay, rare-earth (Re) exchanged USY zeolite, thorough mixing and spray drying the slurry to obtain microspheroidal particles and then subjecting the particles to a step of calcination.
The Zeolite Component
The zeolite used may be any derivatives of faujasite, like NaNH4Y, ReNH4Y. Normal Y type zeolite contained in FCC catalyst undergoes dealumination in the commercial FCC unit, and as a result of this, catalytic activity falls while non-framework alumina contributes to the non-selective catalytic cracking leading to higher coke and dry gas. Hence, the use of USY zeolite with lower unit cell size and high crystallinity is preferred in FCC catalyst formulations.
Preferably, USY zeolite is employed having high hydrothermal stability, with SiO2/Al2O3 ratio of 6.5-8.0. The sodium content present in USY zeolite is preferred to be less than 1% wt. Further, to avoid post preparation rare-earth exchanged step, it is preferred to use rare-earth exchanged USY zeolite. Rare earth source may be single rare-earth chloride or mixture or rare-earth chlorides (of La, Ce, Nd, Pr).
To obtain the rare earth form of USY zeolite, low soda USY zeolite is dispersed in rare-earth chloride solution at elevated temperature of 60-80xc2x0 C. for a period of 20-40 minutes. On completion of exchange, the final product is washed free of chlorides, which on volatile free basis contained 3.8-4% wt. of Re2O3 and about 0.9% wt of Na.
The Pseudoboehmite Component
Preferably, binder grade/pseudoboehmite is used having high crystallinity and crystallite size in the range of 45-60 xc3x85 and having low sodium content, preferably less than 0.1% wt. The alumina is preferred to have good surface area of 200-300 m2/g. The pseudoboehmite alumina with the above properties is required to be gelled by a suitable acid, mineral or organic. Organic acids are preferred in place of mineral acids as chloride, sulfate, and nitrate radicals present in the latter are hazardous to hardware as well as to environment. Gelling character of alumina depends on nature of acid, quantity, and temperature. We have found that in order to obtain an alumina suitable to bind zeolite and clay and make the slurry pumpable, balanced quantity of acid must be used and gelled under controlled conditions. Alumina used in the present investigation has surface area of 260 m2/g, crystallite size of 55 xc3x85, pore radius of 28 xc3x85.
The Polysilicate Component
Polysilicate, either Na stabilized or NH stabilized with low soda content is referred for use in the catalyst of our invention. NH stabilized polysilicate is more preferred than the Na stabilized polysilicate, for the reason of lower sodium level. This ingredient, due to its free flow nature and availability in low soda form, is ideal for creating low acidic matrix in the catalyst in the presence of alumina. Polysilicate with small particle size of 180-250 xc3x85 is preferred for incorporation into the catalyst of our invention. Ammonium polysilicate with 16% SiO2 content and average particle size of 220 xc3x85 is used in the present investigation.
The Clay Component
Clays are used in FCC catalysts as filler, for improving the density and dissipation of heat. The most commonly used clays are laolinite and halloysite. They have a two-layer structure consisting of alternating sheets of SiO4 tetra hedra and AlO6 octahedra. Other clays like montmorillonite, bentonites, etc., have also been cited as substitutes for fillers. Clays for the application in FCC formations are required to be purified and have average particle size of about 2 microns or less. The clay which is used in the present investigation is of kaolinite type with more than 80% fraction below 2 microns and sodium content of less than 0.3 wt %.
The zeolite component present in the catalyst composite is in the range of 5-35 wt %, a preferred range being from 15-30 wt %, modified alumina is in the range of 10-40 wt %, a preferred range being 20-30 wt %. Kaolin clay is present in the range of 0-60 wt %, the preferred range being 0-45 wt %. The residual soda level in the finished catalyst is in the range of 0.2-0.7 wt %, preferred level being less than 0.5 wt %. The rare earth oxide content in the catalyst is in the range of 0.5-2 wt %., the preferred range being 0.8-1.2 wt %. The rare earth metal salts employed can either be the salt of a single rare earth metal or mixture of rare earth metals, such as chlorides consisting essentially of Lanthanum, Cerium, Neodymium with minor amounts of Samarium, Gadolinium, and Yttrium.
The calcined microspheres were tested for attrition resistance. This method measures the attrition at a high constant air jet velocity. The fines were removed continuously from the attrition zone by elutriation into a flask-thimble assembly, which was weighed at intervals. These test conditions were similar to those encountered in hydrocarbon conversion operations. The attrited or overhead catalyst so measured is expressed as the weight percent overhead. Percent attrition is calculated as follows:       Percent  attrition:    ⁢            Gms      ⁢              xe2x80x83            ⁢      overhead      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      5      ⁢      –20      ⁢              xe2x80x83            ⁢      hours      ⁢              xe2x80x83            ⁢      period      xc3x97      100                      50        ⁢                  xe2x80x83                ⁢        gms        ⁢                  xe2x80x83                ⁢                  (          initial          )                ⁢                  xe2x80x83                ⁢        charge            -              gms        ⁢                  xe2x80x83                ⁢        overhead        ⁢                  xe2x80x83                ⁢        in        ⁢                  xe2x80x83                ⁢        0        ⁢        –5        ⁢                  xe2x80x83                ⁢                  hrs          .                      xe2x80x83                    ⁢          period                    
High attrition strength is desirable for retaining the microspheres in the reactor.